Tuesday, September 9, 2008

Snowball Earth and the Cambrian Explosion?

A colleague just sent me a link to this presentation on the SciVee website (a kind of YouTube for scientists):

The genetic response to Snowball Earth: role of HSP90 in the Cambrian explosion
Department of Medicine, University of California

I liked it for a number of reasons. The spoken component was well presented and lucid, so it provided a good advert for this medium. The author also looked at ease on screen. However the slides were too small to read on my laptop and it wasn't clear how or whether you can zoom in on them. 

I also liked the way in which in the author attempts to synthesise evidence from highly diverse sources (steroid receptor and chaperonin biology, the fossil record, palaeo-climatology) into a new theory, which, in a nutshell, states that Snowball Earth (a series of extensive glaciations during the Cryogenian Period that ended around 635 Ma) was important in the evolution of signal transduction proteins in that lowered global temperatures led to a loss of function of molecular chaperones that unmasked otherwise hidden mutations. The author proposes that this might have contributed to the diversification of phyla that preceded the Cambrian explosion.

Do I actually believe him? I tend to be rather sceptical. The whole hypothesis seems too elaborate for its own good. If one wanted to try to explain the diversification of phyla by reference to Snowball Earth, it seems more likely that a rather more mundane explanation might work better, for example a series of repeated population bottlenecks among multicellular organisms that led to a series of marked founder effects (a similar idea has been posited to explain human racial differences on leaving Africa because of a population bottleneck resulting from the eruption of the volcano Toba). 

In addition, the origin of animal phyla (inferred from molecular clock studies) and the Cambrian Explosion (sudden appearance of animals with hard body parts in the fossil record) are in fact different phenomena, which most authorities now seem to believe were separated by a considerable period of time. Much closer in time to Snowball Earth is the emergence of the Ediacaran biota, multicellular organisms that bear little resemblance to modern animal phyla.

But to end on a positive note, it is clear we live in exciting times, as within a few years we will have sequenced the genomes of representatives of all modern animal phyla and in so doing gain fresh insights into their evolutionary origins and diversification. The latest contribution to this effort is the genome of a placozoan, Trichoplax--more information here:

The future is bright, the future is multi-genomic!

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